Designing high-reliability power systems

August 29, 2014 // By Christoph Hammerschmidt
The design of high reliability systems encompasses the use of fault tolerant design techniques, the selection of suitable components to meet the anticipated environmental conditions and compliance to standards. This article focuses on semiconductor solutions for the implementation of high reliability power supplies including redundancy, circuit protection and remote systems management. It will highlight new product features that simplify design and enhance component reliability.

In a perfect world a high reliability system should be designed to avoid single point failures and provide a means of isolating faults in such a way that operation may continue perhaps at a reduced performance level. It should also be able to contain faults to avoid propagation to downstream or upstream electronics.

Built-in redundancy, either in the form of parallel circuits that share the load actively or that wait in a standby until a failure occurs, is one solution. In each case, fault detection and management requires additional overhead circuitry contributing to the overall complexity and cost. Some systems also create dissimilar parallel circuits to add diversity and avoid the risk of a common failure mechanism; this is the case for some aircraft flight control systems.

High complexity systems increase power supply performance requirements and high conversion efficiency and good thermal management are critical as for every 10 °C rise in junction temperature the IC lifetime is approximately halved. As we shall see, new feature rich power supply ICs and dedicated power management functions now provide increased protection to the IC itself and the surrounding system.

Power Regulator Safety Features

Output Current Limiting

This is not a new feature but its implementation has become more accurate and sophisticated and additional flexibility is provided as user programmable features are added. As an example, the LT3667 shown in figure 1 is a 40V 400mA step-down switching regulator with dual fault protected low dropout linear regulators. Internal protection circuitry includes reverse battery protection, current limiting, thermal limiting and reverse current protection. The switching regulator part of the IC contains both a switch current limit and catch diode current limit such that the output current is controlled during fault conditions such as a shorted output. The dual linear regulators also have individual user programmable current limits, which in the example application in Figure 1 have been set to 100mA by R7 and

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